Method for recovery of oil from wells



E. R. BRONNSCOMBE METHOD FOR RECOVERY OF OIL FROM WELLS ,Fiied April 27, 1950 a l" P 6 (2F LE HY $191..

sN A IN VEN TOR. Eugene R. Brownscombo ATTJEST BY 2 L Afirney Patented Sept. 2, 1952 UNITED scum sTATEs PATENT oFFica METHOD FOR RECOVERY OF OIL FROM WELLS Application April g7, 1950, Serial No. 158,386

Claims. 1

This invention relates to a method for producing oil from a subterranean oil reservoir. More particularly, the invention relates to the recovery of oil from a reservoir by means of normally gaseous hydrocarbons at high ressu es.

In the recovery of oil from a subsurface oil reservoir, it has been the practice heretofore to permit the oil to how from the reservoir under the force of the native reservoir energy which may be in the form of a water, gas cap, or gas drive, or combinations thereof. This production, byvirtue of native reservoir energy, is commonly referred to as primary recovery. Further, it has been common practice to augment the native reservoir energy either during the early stages of oil production or more often after the reservoir has approached its economic production limit by primary recovery methods. This may be accomplished by any one or more of several known methods including gas and water injection to provide artificial pressure drives and production in this manner is termed secondary recovery."

In providing a reservoir with such a gas or water drive it is conventional practice to inject the gas or water into the oil producing zone at relatively low pressures of the order of 1,500 p. s. i. Although these methods of secondary recovery have resulted in an increase in the ultimate recovery of oil over that possible solely by'means of primary recovery methods, in some cases only about 20% to 30% of the initial oil in place is recovered and frequently as much as 50% of the oil remains after depletion of the reservoir regardless of the heretofore known method or methods of primary and secondary recovery employed for producing the oil.

In copending United States application, Serial No. 142,568, filed February 6, 1950, now abandoned, by Leonidas P. Whorton and Eugene R. Brownscombe, there is disclosed a method for producing from an oil reservoir whereby a greater amountof oil is recovered than otherwise might be recovered by previously known methods of tion of the method is somewhat restricted by the relatively large volume of normally gaseous hydrocarbons required to practice the same. Thus in areas where large supplies of such gas are not readily available for economic reasons the application of the method is impractical.

The present invention is directed to improvements in the foregoing method whereby the same oil recovery or degree of depletion may be realized by means of relatively small volumes of normally gaseous hydrocarbons. It has been discovered that a greater amount of oil may be recovered from a subterranean oil reservoir than otherwise might be recovered by injecting into the reservoir/ through an injection well normally gaseous" hydrocarbons at a pressure in excess of 3,000,

p. s. i. and in an amount substantially less than one, but more than 0.1 f hydrocarbon pore volume, thereaftei iniecting ingrt quid, hereinafter defined, into the rgsew ir through the same well at ap roximately t he same pressure to cause the normally gaseous hydrocarbons-to pass through the reservoir in contact with the oil containedreservoirs and also artiall depleted reservoirs, provided, however, that the reservoir has not Been depleted to the extent that injected gas would bypass the oil and thereby efiect recovery merely by evaporation into the injected gas and subsequent recovery from the gas by retrograde condensation at the earths surface.

This method is to be distinguished from conventional methods of oil production wherein natural gas is injected into the reservoir at relatively memes of the order of 1,500 p. s. i. for

the purpose of providing a gas drive to augment the natural reservoir pressure. These heretofore known methods of oil production by means of av gas drive differ from the method of the subject invention in that the gas is injected into the reservoir at a pressure much lower than the minimum pressure necessary for carrying out the present invention, and further, in that the gas drive functions merely to sweep oil from the reservoir and does not bring wuwiscosit and reservoir ratio changes necessary to ffillzea substantial increase in the recovery of oil over that obtainable in inert gas or water drives, as does the method of the present invention.

Accordingly, one object of this invention is to provide an improved method for producing from an oil reservoir.

Another object is to provide such a method wherein normally gasegus hydrocarbons in an amount less than one, but more than 0.1, hydrocarbon pore volume, hereinafter defined, are injected into the oil reservoir and thereafter inert fluid is injected thereinto to cause the injected hydrocarbons to pass through the reservoir in contact with the oil, the injection steps being under conditions such that a greater amount of oil is recovered than otherwise might be recovered by conventional methods of primary and secondary recovery.

A further object of this invention is to provide such a method wherein les s than one, but mgge. than 0.1, hydrocarbon pore volume of normally gaseous hydrocarbons is injected into the oil reservoir at a pressure in excess of BQQ O p 's. i thereafter inertrflnidcat substantially the same pressure is injected thereinto to cause the injected hydrocarbons to flow through the reservoir in contact with the oil contained therein, and oil primarily in the liquid phase swept from the reservoir is removed to the earths surface.

Other objects and features of the invention will be apparent from the description and drawing which follow.

The drawing is a diagrammatic view showing a system for carrying out the present invention.

Referring to the drawing, numeral i denotes generally the surface of the earth. Communieating with oil reservoir 2 are injection well 3 and production well 4 spaced from the injection well. These wells may be of a conventional type and. each includes, for example, at least a casing string 5, the interior of which is in communication with reservoir 2 through suitable perforations 6, and a tubing string 1 disposed within the casing for conducting fluids to or from the reservoir.

In accordance with this invention, normally gaseoj g hydrocarbons such as gaseous mixtures able pump or compressor 8, which may be of any conventional type, and are passed through valve controlled pipe 9 into injection well 3 from which the injected gas flows into the reservoir topgn;

tact the oil contained therein. Gas from a conventional oil field separator I0, provided for stabilization of production flowing from reservoir 2 to the Well surface through production well 4, as described below, may be employed as the injection gas, the separator gas entering compressor 8 through flow pipe I I and valve controlled line l2. In practice, however, the volume of separator gas frequently is inadequate, making it necessary to supplement it with gas obtained from a suitable external source, not shown, such as a gas well. If desired, the injection gasmay consist solely of gas from the external source, which gas is conducted to compressor 8. through valve controlled line 13 and flow pipe H Injection of normally gaseous hydrocarbons is continued until a predetermined volume, ascertained in a manner described below, and which in all cases is substantially less than one, but greater than 0.1, hydrocarbon pore volume has been introduced into the reservoir. At that time injection of normally gaseous hydrocarbons is discontinued and injection of an inert fluid islg, sure in excess of 3,000 p. s. i. a predetermined pressure asthe gaseous hydrocarbons. Tfi ert fluid is used herein to denote a fluidotherthi giy w' 5 carbons which has low solubility in the reservoir oil. Included in such term are nitro en ir. and her fliudseu.fi inb;igg. figgg unctionsj to'drivejgrigrcejhe gaseous hydrqga pqn cc iiin reseryni tiie'o l contained therein without decreasing substahtiaiiy the reservoir pressure.

The inert fluid may be obtained from any convenient source and is introduced into pump or compressor 8 through valve controlled line l3 and flow pipe H, the valve .in. line 2 previously having been closed to prevent further introduction of normally gaseous hydrocarbons into the compressor and the valve in pipe 14 having been opened to permit gas to pass from separator i B.

Simultaneously with the injection of gaseous hydrocarbons and inert fluid, the reservoir at preeluetionawell l is maintainedatapressure'"6f"- the sane'orderofiiia'gnitude' as at the injection well 3 but sufficiently below the injecting pressure so as to permit oil to flow through the reservoir. The oil caused to flow through the reservoir by the injected gas is removed to the surface of the earth through production well 4. Most of the oil recovered by this method is in the liquid phase but a small amount flows to the well as a dense vapor phase. In some cases, at pressures in excess -of-4,000 p. 5 e But, which is initially a liquid and ultimatelyiaigas. flows rronrtne reservoir'mto the well as a single W W Injection of inert fluid is continued until oil' and normally gaseous hydrocarbons no longer flow from the reservoir in substantial quantities and injected inert fluid begins to be produced through well 4. At that time recovery of oil in accordance with the present invention is completed.

The oil produced through well 4 flows by means of valve controlled pipe I5 into separator l0, operated at a pressure of 500 p. s. i., for example, less than the pressure of the oil at the well head, wherein the oil is stabilized. As pointed out hereinabove, the separator gas, which comprises normally aseous hydrocarbons, may be conducted by means of flow line 12 into compressor 8 for injection into the reservoir through well 3. However, after injection of gaseous hydrocarbons is discontinued and injection of inert fluid is commenced, as above described, the valve in line i2 is closed and the valve in pipe I4 is opened to permit the overhead gases to pass from the separator. The stabilized oil from separator l0 flows through valve controlled pipe l6 into a second separator 11, which preferably is operated at atmospheric pressure, wherein the oil is stabilized at such pressure. The overhead gas passing through pipe 48 preferably is employed as fuel for the operation of auxiliary field equipment. The stabilized oil is passed from separator I! into a suitable storage tank 19 by means of valve controlled flow line 20.

It has been discovered that with substantially less than one hydrocarbon pore volume of normally gaseous hydrocarbons there may be recovered from a reservoir a greater amount of oil than otherwise might be recovered with conventional methods of "primary and secondary recovery, by injecting into the reservoir at a presvolume of such gas, followed by inert fluid which functions to cause the hydrocarbon gas to pass through the reservoir in contact with the oil contained therein while maintaining the pressure in the reservoir. This increased recovery is begem the liquid some of the oil evaporates into the gas and simultaneously some of the gas dissolves in the oil whereby. the viscosities of the fluids approach one another. As the viscosities of the two fluids become more nearly equal the oil production approaches the condition of displacement of a fluid by fluid of the same composition which would result in complete displacement of the fluid originally in place. It is to be understood, however, that the invention is not limited by any specific theory of operation and that any theory that has been advanced is merely to facilitate the disclosure of the invention.

By hydrocarbqwrgvglu me, as used herein, is meant the volume occupied by reservoir oil in that portion of the reservoir. through which the injected normally gaseous hydrocarbons pass in flowing from the injection well 3 to the production well 4. For example, in the case of a. small reservoir having substantially no channels through which the injected gas may be bypassed through only a small .portion of the reservoir and with the injection well and production well communicating with the reservoir at opposite sides thereof, the hydrocarbon pore volume is equal substantially to the volume of the reservoir occu- -pied by all of the oil. In cases where the reservoir is relatively large and the injection and production wells are located at one end of the reservoir the hydrocarbon pore volume is that volume of the reservoir occupied by the reservoir oil through which the injected normally gaseous hydrocarbons actually pass in moving from the in jection well 3 to the production well 4. The hydrocarbon pore volume may be determined sufllciently close for the purpose of this invention in accordance with procedures well known in the industry from certain information obtained by established investigating methods. Such procedures are so well known in the art that a description thereof is unnecessary.

As above mentioned, prior to injection of the inert fluid there is introduced into the reservoir a predetermined volume of normally gaseous by;

drocarbons, which is less tha'rf'one, but greater ,tssrroi, hydrocarbon pore volume. Such volwas 'may be ascertained by laboratory experiments, below described, on a core under conditions simulating recovery of oil from a subterranean oil reservoir. The core may comprise either a synthetic core formed of sand or other suitable material, such, for example, as carborundum, or a coresample obtained from the reservoir by conventional coring methods.

The following laboratory experiments exemplify a method and apparatus for determining the volume of normally gaseous hydrocarbons which has characteristics approaching those of the subterranean reservoir. The segments preferably are joined by flat, narrow rings, the whole of the core sample being sealed within a rubber sheath by applying su-flicient fluid pressure in the annulus between the sheath and metal tubing thereby reducing to a minimum channeling along the surface of the core sample. A 26 liter pressure vessel used for storing the reservoir oil to be displaced is connected by means of a pipe to one end of the artificial reservoir and a suitable pump is connected in the pipe for transferring to the reservoir high pressure fluids. The normally gaseous hydrocarbons to be injected into the simulated reservoir are stored in a high pressure vessel having a capacity of about 30 liters and which preferably is connected by a suitable line to the same end of the reservoir as is the 26 liter storage cell hereinbefore mentioned. To the other end of the simulated reservoir a single pipe is connected for conducting effluent from the reservoir to a high pressure visual separator which is of any conventional design. The function of the visual separator is to permit observation of the reservoir effluent whereby a determination may be made, if desired, of whether the efiluent flowing from the reservoir at any instant is a liquid or gas or a mixture thereof. From the visual separator the reservoir efliuent is conducted by means of avalve controlled pipe into a second separator operated at atmospheric pressure wherein the eflluent is stabilized. The stabilized liquid and the gaseousfraction from the atmospheric separator are metered to determine the volume thereof. The two fractions are analyzed periodically to determinethe compositions thereof from which the number of hydrocarbon pore volumes of injected gas in the efiluent may be determined, knowing the compositions of the injected gas and original reservoir oil.

The laboratory experiments with the above described apparatus are conducted in the following manner. Oil which has been stabilized at atmospheric pressure, and which is referred to hereinafter as dead oil, is pumped into the simulated core or reservoir under a pressure of 500 p. s. i., for example, to displace air or gas or other fluid which might be present. By means of the heating coil the core is brought up to the desired reservoir temperature. The reservoir oil is then .pumped into the core from the 26 liter storage vessel until all of the dead oil has been displaced and the core is at the desired reservoir pressure, such as 4,000 p. s. i. for example, the amount of dead oil displaced being equal to one hydrocarbon pore volume. At this time injection of normally gaseous hydrocarbons is commenced by displacement of such gas from the 30 liter vessel with water. As pointed out hereinabcve, the gas is injected at a pressure in excess of 3.000 p. s. i. By maintaining the effluent end of the core at a pressure somewhat lower than the injection pressure, such, for example, about 50 p. s. i. lower, the injected gas is caused to flow through the core forcing oil therefrom. The oil passes through the visual separator and into the atmospheric separator wherein it is stabilized. Gas

injection is continued until the gas-oil ratio of hydrocarbon pore volume of injected gas. ever, only 0.20 hydrocarbon pore volume was provolumes of the subterranean reservoir must be injected into such reservoir in order to recover oil therefrom in accordance with this invention.

' The following examples of results obtained in e the laboratory on an artificial reservoir by experiments conducted as above described, illustrate the quantity of normally gaseous hydrocarbons required to, be injected into and forced by inert gas through a subterranean reservoir, having characteristics similar to the artificial reservoir employed, to recover oil in accordance with this invention.

Example I Employing injection gas and reservoir oil of the following compositions, at an injection pressure of 4,000 p. s. i. and with the reservoir at 140 F. a recovery of 88% of the original oil in place was realized utilizing 1.31 hydrocarbon pore volumes of injected gas. However, only 0.43 hydrocarbon pore volume was produced and appeared in the eiliuent, this amount of injection gas being all that is necessary to effect the recovery. Thus, normally gaseous hydrocarbons of the same composition and in an amount equal to 0.43 hydrocarbon pore volume of the subterranean reservoir are required to produce oil of .the same composition from such reservoir under similar conditions of temperature and pressure.

Total Example II Employing injection gas and reservoir oil of the following compositions, the reservoir oil being of the same composition as that employed in Example I, at the same injection pressure and reservoir temperature, a recovery of 80% of the original oil in place was realized utilizing 1.00

How-

duced and appeared in the eilluent, this amount of injection gas being all that was necessary to efi'ect the recovery. Thus, normally gaseous hydrocarbons in an amount equal to 0.20 hydrocarbon pore volume of the subterranean reservoir are required to produce oil of the same composition from a subterranean reservoir having characteristics similar to the artificial reservoir and under the same conditions of temperature and pressure, in accordance with the invention.

By comparison of the two examples it will be observed that the composition of injected gas afiects not only the recovery but also the injection gas requirements. Whereas economics might dictate use of lean gas, such as a mixture of methane and ethane with a small amount of heavier hydrocarbons for injection into the reservoir, nevertheless it is possible to bring about a further increased recovery by use of a rich gas, 1. e., a normally gaseous hydrocarbon mixture containing a substantial amount of propane and butane and heavier hydrocarbons. Similarly, a greater recovery may be realized at injection pressures of 6,000 p. s. i. and higher than at 3,500 p. s. i., but because of the relatively high costs involved in repressuring gas it may be preferred to inject the gas at about 4,000 p. s. i.

An alternate method for determining the volume of normally gaseous hydrocarbons which must be injected into the reservoir before injection of inert fluid for recovering oil in accordance with the present invention, and which is preferred in cases where water is to be employed as the inert fluid, contemplates use of the same apparatus as the above method and is substantially the same as the latter except that a given amount of normally gaseous hydrocarbons, such, for example, as 0.1 hydrocarbon pore volume, is injected into the artificial reservoir and thereafter inert fluid is-injected thereinto until the gas-oil ratio of the efiluent is equal to 30,000 where the inert fluid is a gas or until water in substantial quantities appears in the eflluent where the inert fluid is water. The oil recovery is then determined. The foregoing procedure is re-- peated several times using progressively larger volumes of normally gaseous hydrocarbons. When the recoveries are plotted as a. function of the quantity of normally gaseous hydrocarbons employed it will be noted that the recovery gradually increases with increase in quantity of injected normally gaseous hydrocarbons up to a certain fraction of a hydrocarbon pore volume and thereafter remains substantially constant. This is the amount of normally gaseous hydrocarbons which must be injected into a reservoir in order to recover oil therefrom in accordance with the present invention.

This invention, as above mentioned, contemplates the injection, into the reservoir at a pressure in excess of 3,000 p. s. i., of normally gaseous hydrocarbons in an amount less than one, and greater than 0.1, hydrocarbon pore volume. The quantity of hydrocarbon gas requiredis infiuenced by various factors such, for example, as compositions of injection gas and reservoir oil, injection pressure, and the composition of inert fluid. The volume required, however, is determined, as above described by laboratory experiments on an artificial reservoir.

While in the foregoing there has been shown and described the preferred embodiment of this invention, it is to be understood that minor changes in the details of construction, combination, and arrangement of parts may be resorted to without departing from the spirit and scope of the invention as claimed. For example, whereas only a single injection and production well are shown, two or more of such wells may be utilized if desired.

I claim:

1. A method for recovering oil from a subterranean oil reservoir containing oil in quantities such that gas injected thereinto will not substantially bypass the oil, and in communication EXAMii'itii with which are an injection well and a production well spaced from the injection well, comprising the steps of injecting into the reservoir through the injection well normally gaseous hydrocarbons at a pressure between about 3,000 p. s. i. and 6,000 p. s. i. and in an amount substantially less than one hydrocarbon pore volume, thereafter injecting inert fluid into the reservoir through the injection well at a pressure between about 3,000 p. s. i. and 6,000 p. s. i. to cause the normally gaseous hydrocarbons to pass through the reservoir in contact with the oil contained therein, simultaneously with the injecting steps maintaining the reservoir pressure at the production well substantially of the same order of magnitude but sufiiciently below the injecting pressure so as to permit oil to flow from the reservoir, and removing to the earth's surface through the production well oil swept from the reservoir substantially free from any separate gas phase.

2. A method for recovering oil from a subterranean oil reservoir containing oil in quantities such that gas injected thereinto will not substan tially bypass the oil, and in communication with which are an injection well and a production well spaced from the injection well, comprising the steps ofv injecting into the reservoir through the injection well normally gaseous hydrocarbons at a pressure between about 3,000 p. s. i. and 6,000 p. s. i. and in an amount substantially less than one, but more than 0.1, hydrocarbon pore volume, thereafter injecting inert fluid into the reservoir through the injection well at a pressure between about 3,000 p. s. i. and 6,000 p. s. i. to cause the normally gaseous hydrocarbons to pass through the reservoir in contact with the oil contained therein, simultaneously with the injecting steps maintaining the reservoir pressure at the production well substantially of the same order of magnitude but sufficiently below the injecting pressure so as to permit oil to flow from the reservoir, and removing to the earths surface through the production well oil swept from the reservoir substantially free from any separate gas phase.

3. A method for recovering oil from a subterranean oil reservoir containing oil in quantities such that gas injected thereinto will not substantially bypass the oil, and in communication with which are an injection well and a production well spaced from the injection well, comprising the steps of injecting into the reservoir through the injection well normally gaseous hydrocarbons at a pressure between about 3.000 p. s. i. and 6,000 p. s. i. and in an amount substantially less than one, but more than 0.1, hydrocarbon pore volume, thereafter injecting water into the reservoir through the injection well at a pressure between about 3,000 p. s. i. and 6,000 p. s. i. to cause the normally gaseous hydrocarbons to pass through the reservoir in contact with the oil contained therein, simultaneously with the injecting steps maintaining the reservoir pressure at the production well substantially of the same order of magnitude but sufiiciently below the injecting pressure so as to permit oil to flow from the reservoir, and removing to the earths sur- 10 face through the production well oil swept from the reservoir substantially free from any separate gas phase.

4. A method for recovering oil from a subterranean oil reservoir containing oil in quantities such that gas injected thereinto will not substantially bypass the oil. and in communication with which are an injection well and a production well spaced from the injection well, comprising the steps of injecting into the reservoir through the injection well normally gaseous hydrocarbons at a pressure between about 3,000 p. s. i. and 6,000 p. s. i. and in an amount substantially less than one, but more than 0.1, hydrocarbon pore volume, thereafter injecting nitrogen into the reservoir through the injection well at a. pressure between about 3,000 p. s. i. and 6,000 p. s. i. to cause the normally gaseous hydrocarbons to pass through the reservoir in contact with the oil contained therein, simultaneously with the injecting steps maintaining the reservoir pressure at the production well substantially of the same order of magnitude but sufficiently below the injecting pressure so as to permit oil to flow from the reservoir, and removing to the earth's surface through the production well oil swept from the reservoir substantially free from any separate gas phase.

5. A method for recovering oil from a subterranean oil reservoir containing oil in quantities such that gas injected thereinto will not substantially bypass the oil, and in communication with which are an injection well and a production well spaced from the injection well, comprising the steps of injecting into the reservoir through the injection well normally gaseous hydrocarbons at a pressure between about 3,000 p. s. i. and 6,000 p. s. i. and in an amount substantially less than one, but more than 0.1, hydrocarbon pore volume, thereafter injecting air into the reservoir through the injection well at a pressure between about 3,000 p. s. i. and 6,000 p. s. i. to cause the normally gaseous hydrocarbons to pass through the reservoir in contact with the oil contained therein, simultaneously with the injecting steps maintaining the reservoir pressure at the production well substantially of the same order of magnitude but sufiiciently below the injecting pressure so as to permit oil to flow from the reservoir, and removing to the earth's surface through the production well oil swept from the reservoir substantially free from any separate gas phase.

EUGENE R. BROWNSCOMBE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. A METHOD FOR RECOVERING OIL FROM A SUBTERRANEAN OIL RESERVOIR CONTAINING OIL IN QUANTITIES SUCH THAT GAS INJECTED THEREINTO WILL NOT SUBSTANITALLY BYPASS THE OIL, AND IN COMMUNICATION WITH WHICH ARE AN INJECTION WELL AND A PRODUCTION WELL SPACED FROM THE INJECTION WELL, COMPRISING THE STEPS OF INJECTING INTO THE RESERVOIR THROUGH THE INJECTION WELL NORMALLY GASEOUS HYDROCARBONS AT A PRESSURE BETWEEN ABOUT 3,000 P. S. I. AND 6.000 P. S. I. AND IN AN AMOUNT SUBSTANTIALLY LESS THAN ONE HYDROCARBON PORE VOLUME THEREAFTER INJECTING INERT FLUID INTO THE RESERVOIR THROUGH THE INJECTION WELL AT A PRESSURE BETWEEN ABOUT 3,000 P. S. I. AND 6,000 P. S. I. TO CAUSE THE NORMALLY GASEOUS HYDROCARBONS TO PASS THROUGH THE RESERVOIR IN CONTACT WITH THE OIL CONTAINED THEREIN, SIMULTANEOUSLY WITH THE INJECTING STEPS MAINTAINING THE RESERVOIR PRESSURE AT THE PRODUCTION WELL SUBSTANTIALLY OF THE SAME ORDER OF MAGNITUDE BUT SUFFICIENTLY BELOW THE INJECTING PRESSURE SO AS TO PERMIT OIL TO FLOW FROM THE RESERVIOR, AND REMOVING TO THE EARTH''S SURFACE THROUGH THE PRODUCTION WELL OIL SWEPT FROM THE RESERVOIR SUBSTANTIALLY FREE FROM ANY SEPARATE GAS PHASE. 